Numerous polymer-based medical devices have been developed for implantation or insertion into the body. For example, in recent years, drug eluting coronary stents, which are commercially available from Boston Scientific Corp. (TAXUS and PROMUS), Johnson & Johnson (CYPHER) and others, have been employed for maintaining vessel patency. These existing products are based on metallic expandable stents with polymer coatings, which release anti-restenotic drugs at a controlled rate and total dose. Specific examples of polymers for drug eluting polymer coatings include various copolymers such as poly(ethylene-co-vinyl acetate), poly(styrene-b-isobutylene-b-styrene) triblock copolymers (SIBS), described, for instance, in U.S. Pat. No. 6,545,097 to Pinchuk et al., and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), described, for instance, in Pub. No. 2005/0106204 to Hossainy et al.
In one process, the outer surface of a stainless steel coronary stent is sprayed first with a solution that contains poly(n-butyl methacrylate) as an adhesion promoting coating, and then with a solution that contains PVDF-HFP and everolimus as a drug eluting coating. See, e.g., Pub. No. 2005/0106204 to Hossainy et al. The solutions are sprayed on the outside of the stent, and to some degree, through the stent struts. The stent struts are ultimately encapsulated with the two coatings due to a combination of outside spraying and through-strut spraying combined with flow of the solution around the stent struts. The net result is that the spray process results in a conformal coating.
The result of such a process is schematically illustrated in FIGS. 1A and 1B. FIG. 1A shows a stent 100 which contains a number of interconnected struts 100s. FIG. 1B is a cross-section taken along line b-b of strut 100s of stent 100 of FIG. 1A, and shows a stainless steel stent substrate 110, an adhesion promoting coating 120, and a drug eluting coating 130. Because the drug eluting coating 130 encapsulates the adhesion promoting coating 120 and the substrate 110, the coating 120 does not need to be particularly efficacious at promoting adhesion between the coating 130 and substrate 110, so long as the coating 130 has good mechanical integrity.